Printed circuit boards are commonly used to mount and interconnect electronic devices. Circuit boards are generally made from circuit board laminate comprised of a reinforced polymeric material. If subtractive techniques are used to form the interconnection circuitry, the laminate is clad with copper foil or some other suitable conductive material. Different reinforcements are used, including paper and fibers of glass, quartz, aramids, and graphite. The fibers may be used in woven or non-woven form.
The circuit boards used in advanced electronic applications must meet demanding requirements in order to allow for proper performance. The polymers used in the circuit board laminate must have adequate thermal, electrical and physical properties. In high performance applications, properties such as a high glass transition temperature (Tg), a low thermal expansion coefficient, a low dielectric constant and moisture resistance are desirable.
It is also desirable that the processing characteristics of the polymer be suited to conventional processes used in the circuit board industry. Circuit board laminate is typically fabricated from prepregs which comprise reinforcement impregnated with a partially cured prepolymer. Prepreg is typically fabricated using a solution impregnation process. The reinforcement (typically fabric) is passed in a continuous manner through a prepolymer solution and then through an oven in which the solvent is removed. The prepolymer may also be partially cured (advanced) in this oven.
Single and double sided circuit boards make use of circuitry on one or both sides of the laminate. Typically, laminates are formed by stacking individual plies of prepreg and laminating them under heat and pressure in a hydraulic press. If circuitry is to be formed via a subtractive process, a conductive layer, typically copper foil, is laminated to one or both sides of the laminate during this operation. The polymeric portion of the laminate is cured during lamination. Post cure at an elevated temperature may be required to more fully cure the polymer.
Multilayer circuit boards contain circuitry on external and internal layers. Circuitry is formed on one or both sides of thin laminates. These are then bonded together in a second lamination step using prepreg as an adhesive dielectric layer.
A proper prepolymer content (weight percent prepolymer) must be achieved on the prepreg. The prepolymer content is influenced by factors such as the solids content of the solution, the mechanical arrangement of the impregnation device (called a treater), and the viscosity of the resin. Depending on the nature of the reinforcement being used, desirable prepolymer contents may range from about 35% to about 75%. For each specific reinforcement, such as a specific style of woven glass cloth, the prepolymer content must be controllable to within several percent of a specific value in order to allow for proper performance in subsequent processing steps and during circuit board operation. The polymer serves as a dielectric layer and, therefore, there must be sufficient polymer between conductive layers so that the board does not fail during operation. In some instances, the thickness of this dielectric layer affects the electrical performance of the circuit board. Control of prepolymer content is required to control the thickness of the dielectric layer.
It is desirable in many, but not all, instances that circuit board prepreg be substantially tack-free in order to facilitate handling and avoid contamination. With some compositions, simply removing the solvent during prepreg fabrication yields tack-free prepreg. With others, advancement is required to yield tack-free prepreg.
The prepolymer on the prepreg must also be capable of flowing during lamination. Its melt viscosity must be low enough during lamination to allow for consolidation of the individual prepreg plies into a unified laminate and to allow for elimination of trapped air or volatiles, but not so low that excessive flow occurs. Proper prepolymer flow is of similar importance when a multilayer circuit board is fabricated.
The inventors studied use of the polymers and prepolymers made from polycyclic polyenes (such as dicyclopentadiene) and cyclic siloxanes (such as methylhydrocyclosiloxanes), as described by Leibfried in U.S. Pat. Nos. 4,900,779 and 4,902,731, for preparation of prepreg and laminate for printed circuit boards. They discovered that processing characteristics are enhanced by use of a chain extender (component (a) described below), and that hydrosilation compositions comprising a chain extender make excellent prepolymers, prepregs, laminates and polymers for printed circuit boards.